1. Field of the Invention
The invention is in the field of reflecting devices of the retroreflector type in which at least one of the sequence of reflective surfaces, forming the retroreflector, is activated to superpose a doppler shift on the return signal reflected therefrom toward the transmitter of the original entrance signal.
2. Description of the Prior Art
Passive retroreflective devices for electromagnetic radiation, in contrast to active responders, return an essential part of the received energy back into the direction of its origin. The most common types of passive retroreflectors are found in the optics of traffic warning signals and consist of optical retroreflector elements such as corner reflectors, cat's eye lenses, or of arrays made of these reflector elements. For the microwave radio region, one or several elements of such arrays may suffice for effectively returning most of the incoming radiation if the elements are much enlarged. The efficiency of any reflector element depends primarily on the reflectance of its active surface area and on its gross aperture. It has become practicable in testing operations to compare the reflected energy from a target with that from a large metal sphere of a specified radius. In the 10.sup.9 Hz (.lambda.=3 cm) region, for instance, the return radiation from a corner reflector of 10 cm aperture corresponds roughly with that of a reflecting sphere of about 10 meters radius. The amount of energy reflected from such a device is of the same order of that reflected from the entire front end or rear end of ordinary motor vehicles.
In true-range power tests, the present state of technology leans heavily on stationary corner cube reflectors, but if doppler-shifted signals are to be tested or superposed upon the search beam an auxiliary target is usually placed quite close to the transmitter and is moved back and forth in beam direction. Accordingly, doppler-testing is performed in the transmitter's near field, where signal power levels are often excessive and where minute reflecting components often suffice. However, true-range testing and target searches will always require precise and synchronous tracking of all movements and speeds of the target object by other means also.
Prior testing art for small doppler-shifts consists of using tower mounted corner reflectors for beam and target marking wherein there was no simple doppler-shifting device available. Operators also use active frequency shifted responders for microwave link testing. However, most of these responders are not at all designed for mobile target simulations. Another testing method that is used employs on-board vehicle speed-scribers and recorders for such telemetry projects and their subsequent motion analysis. In testing and other activities of modern motion-detecting radar devices, the combination of the measured energy level with true-range indication of the returning signal, together with a predetermined superimposed doppler-shift of known magnitude, has now become a very desirable measurement feature. This aspect of true-range mobile target tests has remained little-explored. It is one purpose of this invention to combine ordinary passive retroreflecting testing components with the option to superimpose on the primary beam selected signals doppler shifts of a known magnitude for simulating the movement of a distant target together with its indicated range.